This invention relates to a process for upgrading a naphtha, e.g., straight run naphtha, catalytically cracked naphtha (FCC gasoline), etc., and mixtures thereof, in a multiple riser fluid catalytic cracking operation utilizing a mixture of amorphous cracking catalyst and/or large pore crystalline cracking catalyst, e.g., zeolite Y, and a shape selective medium pore crystalline silicate zeolite catalyst, e.g., zeolite ZSM-5.
In known and conventional fluidized catalytic cracking processes, a relatively heavy hydrocarbon feedstock, e.g., a gas oil, admixed with a suitable cracking catalyst, e.g., a large pore crystalline silicate zeolite such as zeolite Y, to provide a fluidized suspension is cracked in an elongated reactor, or riser, at elevated temperature to provide a mixture of lighter hydrocarbon products. The gasiform reaction products and spent catalyst are discharged from the riser into a separator, e.g., a cyclone unit, located within the upper section of an enclosed stripping vessel, or stripper, with the reaction products being conveyed to a product recovery zone and the spent catalyst entering a dense catalyst bed within the lower section of the stripper. In order to remove entrained hydrocarbon product from the spent catalyst prior to conveying the latter to a catalyst regenerator unit, an inert stripping gas, e.g., steam, is passed through the catalyst where it desorbs such hydrocarbons conveying them to the product recovery zone. The fluidized catalyst is continuously circulated between the riser and the regenerator and serves to transfer heat from the latter to the former thereby supplying the thermal needs of the cracking reaction which is endothermic.
Particular examples of such catalytic cracking processes are disclosed in U.S. Pat. Nos. 3,617,497, 3,894,932, 4,051,013, 4,309,279 and 4,368,114 (single risers) and U.S. Pat. Nos. 3,748,251, 3,849,291, 3,894,931, 3,894,933, 3,894,934, 3,894,935, 3,926,778, 3,928,172, 3,974,062 and 4,116,814 (multiple risers).
In accordance with the fluid catalytic cracking process of U.S. Pat. No. 4,051,013, a naphtha feed and a gas oil feed are converted in the presence of an amorphous or zeolite cracking catalyst, preferably the latter, in a riser reaction zone to provide a high octane gasoline.
Several of these processes employ a mixture of catalysts having distinctly different catalytic properties as, for example, the dual riser catalytic cracking process described in U.S. Pat. No. 3,928,172 which utilizes a mixture of a large pore crystalline silicate zeolite cracking catalyst, e.g., zeolite REY, and a shape selective medium pore crystalline silicate zeolite, e.g., ZSM-5. According to this process, a gas oil is cracked in a first riser in the presence of the aforesaid catalyst mixture, principally as a result of the zeolite REY component thereof, to provide a heavy naphtha product, the latter being recracked and/or a virgin straight run naphtha being cracked in a second riser in the presence of the catalyst mixture to provide a high octane gasoline product together with varying quantities of C.sub.3 and C.sub.4 olefins.